Videos are often changed from an original video stream to a modified one. The impetus to change is often related to the bandwidth of the target medium over which the modified video stream will be transmitted, but there are a variety of reasons to modify videos. Other reasons for processing video include editing, compression and decompression, reformatting, video insertion and overlay, minimization of transmission errors, and modifying color for different displays, for example.
Some systems for comparing two videos to one another include two inputs, one of a video being tested and one as a reference video. One such system is the PQA product line such as the PQA 500/600, or PQASW available from Tektronix of Beaverton, Oreg. The two videos are directly compared and various measurements are generated describing the original, modified, or the comparison between them. Other systems for comparing two videos include showing an original video first, then quickly showing the modified video, or vice-versa. Not only is such a system time consuming, such comparisons are not as accurate as those simultaneously made due to unaccounted human color memory distortions and fading of color memory inherent in human viewers. Successive viewing is the dominant, and may be only, feasible way to compare video as completely as possible for the most repeatable and reliable video quality assessment by humans.
The effects of color bias when evaluating images recalled from memory occur in several scenarios. For example, video data compression and decompression (codec) technology is inherently lossy, with errors introduced. The best technology introduces error with minimum perceptual impact. However, even a very good codec technology that produces good looking video can produce ugly artifacts if certain video content is used and/or the bit rate selected is set too low. Color artifacts are among the artifacts that can be created. The industry standard best method of evaluating subjective quality is ITU-R BT.500, which describes many methods of using people to view and rate the video, and then getting statistics to indicate the best among those evaluated. Generally short video clips of 8 to 12 seconds are used and rated successively. The shortest time between clips is generally about a couple of seconds, so that the shortest time between corresponding frames is about 10 seconds, while the longest time between corresponding frames could be minutes. Though ITU-R BT.500 is followed by many engineering organizations, in many cases it is too expensive and/or time consuming. In these cases, individual evaluators may simply look at one clip, then the other. In all of these cases, memory of color is used in the evaluation, with delay between corresponding frames ranging from a few seconds to many seconds to minutes to indefinite (i.e. no reference used, simply using memory of how similar objects look from previous experience).
Another scenario regarding color analysis from recalled colors involves video conversion between resolutions and frame rates. For example, many converters have been sold specifically for evaluating and/or optimizing conversion from HD to SD and from SD to HD. The digital representation of color is different in HD than historic SD, though the US standard for digital television transmission (ATIS) allows for both old and new methods (SMPTE-M vs. 709 colorimetry) for SD resolution. In various audits of broadcast signals, it was common to find incorrect colorimetry being broadcast due to conversion, or simply due to not setting the proper flags for colorimetry in the corresponding data field. While the resulting color error is often subtle, it can be quite obvious in some cases. Again, the evaluation methods are as in the codec evaluation case, though in practice successive viewing, once each, is often used, especially if it is cumbersome to switch between video standards being displayed.
In a scenario involving editing, when putting together film or video clips from a particular outdoor scene that was shot over a time period spanning several hours, the lighting changes due to change in position of the sun, clouds, etc. For the scene to look coherent, color editing is generally required. In some cases, due to time constraints or due to controls available within given color editing tools, it is not practical or feasible to completely color correct one clip to match another. However, if these two clips are shown with a 3rd clip in between, the resulting delay between the first 2 clips reduces most people's ability to notice a color difference. Being able to predict how many seconds are required (which depends on the type of clip) in between the two clips for matching perceived (memory and direct) colors would save valuable time. Otherwise, educated guesses along with trial and error are used.
Unfortunately, because of the human color memory distortions and fading inherent in human viewers, accurate prediction of color differences is difficult or impossible between a present video being compared to one recalled from memory, even if the recall period is relatively short.
Embodiments of the invention address this and other limitations of the prior art.